GEOCHEMISTRY. — From γῆ, “earth,” and χυμική, “chemistry.” It is the science that studies the physico-chemical composition of the earth. Ever since, in an infinitely remote past, a fragment of solar matter broke away from the intensely hot mass of the star of our system to form the earth, an infinite series of chemical and physico-chemical reactions have agitated the chemical elements present in the solar matter; various compounds have been formed and have disappeared; immense masses have passed from gaseous to liquid, then to solid states, and then, in part, have been remelted; the gases of the primordial atmosphere have reacted with solids and liquids; the first solutions have leached the solid masses and have attacked them; plant and animal organisms, which appeared very recently in the play of reactions, have then contributed to the formation of the present equilibrium. And the reactions continue: even now entire mountain chains are slowly destroyed by the action of geological agents, new sediments are forming within the waters, while lava masses cool and the degassing of magmas continues.
Now, geochemistry investigates all these chemical and physico-chemical reactions that constitute the present and past life of our earth, reactions that describe the genesis of our planet and its current physico-chemical state and, in part, allow us to foresee the future development of the chemical processes that will take place upon II.
Geochemistry is rightly considered a science of the 20th century: it was born from the trunk of geological doctrines to which it belongs and indissolubly links mineralogy and geology to chemistry, reestablishing those relationships that, during the most markedly naturalistic period of geo-mineralogical sciences, seemed to have faded.
The theories that, until recent years, guided the investigations of geochemists assumed that, during cooling, the primitive matter differentiated by gravity, causing a nickel-iron alloy, nife (from Ni = nickel and Fe = iron), to sink toward the center, supposed to constitute the earth’s core or siderosphere, and depositing above it, first a layer of sulfides or calosphere, and then the surface layer or lithosphere, consisting mainly of silicates. The lithosphere, only 30–80 km thick, was divided into two portions: the sima (from Si = silicon and Mg = magnesium), formed by basic rocks, i.e., rich in iron and magnesium silicates, and the sial (from Si and Al = aluminum), lighter, with aluminum silicates and free silica, essentially formed, that is, of rocks of the granite type.
Consequently, with this division of the earth into layers, the elements were also divided into atmophiles, lithophiles, chalcophiles, and siderophiles, depending on whether they were abundant in the atmosphere or in one of the earth’s layers.
The hypothesis regarding the constitution of the deep layers had been reached by considering the difference between the average density of the earth and that of surface rocks, the composition of meteorites, and the propagation of seismic waves.
Now geochemists are orienting themselves toward a new theory, more in line with current knowledge. According to this theory, the matter detached from the sun began, upon cooling, to differentiate, immediately losing large quantities of hydrogen, helium, and other light elements, thus becoming enriched at the surface with more penetrating elements, while new masses of gaseous matter arrived at the surface, stirring the differentiated matter. Further cooling led to the condensation of the less volatile masses and to the formation of a surface liquid shell consisting of ferro-magnesian silicates and having, overall, the chemical characteristics of an alkalized magma. It should be noted that, according to this theory, the degassing and weighting of the surface masses caused them to sink, but only to a depth compatible with the viscosity of the matter, which, approaching the center, was increasingly complex and dense and therefore more viscous. Thus, beneath the liquid shell remained a core of undegassed, supercompressed pristine solar matter.
No general differentiation by gravity or by fractional crystallization has therefore occurred, and thus the present granitic masses would have formed not by differentiation of the primitive magma but by remelting of sediments.
Indeed, above the liquid shell was the pre-geological atmosphere, consisting of water vapor, nitrogen, hydrofluoric, hydrochloric, and carbonic acids, and alkaline chlorides and fluorides, all perhaps at a pressure of 400 atmospheres. The further lowering of the temperature led to the consolidation of the magmatic mass, while pneumatolytic reactions formed deposits with acidic silicates and quartz; below the critical point of water, solutions rich in acids began to act, attacking the surface shell and determining the formation of clayey and siliceous sediments. It is these silica-rich masses, formed in the pneumatolytic and then hydrothermal phases, which, accumulated by waters in depressions and then, through subsidence, found themselves in zones of higher temperature, were remelted and gave rise to many of the present granites.
Investigations into the solar atmosphere show that in it, for every 100 silicon atoms, there are: hydrogen 150,000–300,000 atoms, helium 9,000, oxygen 5,000–10,000, carbon 1,500–3,000, nitrogen 500–1,000, fluorine 500–1,000, magnesium 100–1,000, iron 84, sodium 82, potassium 24, manganese 7, nickel 4.8, cobalt 2.9, chromium 2.2, boron 1, aluminum 0.5–3, calcium 0.4–1.8, vanadium 0.5, zinc 0.3, copper 0.2, titanium 0.03–0.25; all other elements are present in much smaller quantities. The main constituents of solar matter, excluding those gaseous under our environmental conditions, are all found on earth, while, broadly speaking, the minor constituent elements of the sun are also those of the lithosphere.
The first quantitative investigations, based on numerous analyses of surface rocks, have shown that only eight chemical elements (oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium) participate in the formation of the surface crust with weight percentages greater than 1% (among them, oxygen constitutes 49.5% by weight of the crust and silicon 25.3%); four elements (hydrogen, titanium, chlorine, and phosphorus) participate with percentages between 0.1 and 0.9%, and only another thirteen (manganese, carbon, sulfur, barium, chromium, nitrogen, fluorine, zirconium, strontium, nickel, vanadium, zinc, copper) with percentages between 0.01 and 0.1%. The vast majority of elements are present only in traces on the surface, for the sum of the percentages of the aforementioned twenty-five elements amounts to 99.96%. If one then considers the volume that the atoms (or ions) occupy in the crystals of rock minerals, one arrives at the unexpected result that oxygen alone occupies 91.77% of the volume of the lithosphere.
The detailed study of the distribution of elements on our planet can be divided into three parts: the first process of distribution occurred during the formation of the first high-pressure atmosphere and the surface molten crust, impressing, at the center of the mass, pristine supercompressed solar matter; it essentially led to the separation from surface matter of helium, hydrogen, carbon, and nitrogen and to the fixation of large quantities of oxygen in the silicates and oxides of the liquid mass and in water vapor.
In the process of geochemical distribution of the elements, which occurred during the consolidation of the surface molten mass, all the minerals (almost always silicates) of the fundamental constituents of the magma (aluminium, iron, calcium, sodium, potassium, and magnesium) separated out, and the minor constituents (the other chemical elements of the lithosphere) divided themselves approximately into two categories: those whose size (ionic or atomic radius) allowed them to be accommodated within the principal minerals of the forming rocks (i.e., they could enter their crystals as isomorphic substituents of the fundamental constituents) and which thus became diluted in the solid mass during its crystallization; and the others, the free elements, which could not enter the crystals of the common minerals and which accumulated in the residues of the crystallizations, giving rise to their own minerals in the pegmatitic-pneumatolytic or hydrothermal phase.
Finally, a third process of geochemical distribution of the elements occurs during the transformations that the minerals of the lithosphere have undergone and are undergoing during the processes of attack and transport by geological agents or during metamorphism.
These problems of the chemical constitution of the Earth and of the distribution of the elements are fundamental to geochemistry, and here they can only be mentioned. Many other problems are connected with them, including those of bio-geochemistry, i.e., investigations into the cycles that chemical elements undergo when they become part of animals and plants and their vital processes; those concerning the distribution and transformation of radioactive substances; and those concerning the conditions of genesis of individual minerals and then of rocks.
Vedi also the series of works published by V. M. Goldschmidt and collaborators in *Nachrichten der Gesellschaft der Wissenschaften zu Göttingen. Fachgruppe IV (Mineralogie und Geologie)*, 4 (1930), 6 (1930), 11 (1931), 12 (1931), 16 (1931), 18 (1932), 20 (1932), 25 (1932), 26 (1932), 27 (1932), 31 (1932), 33 (1933), 35 (1933), 36 (1933), 37 (1933), 40 (1933), 2 (1934), 4 (1934), 11 (1935), 15 (1936). F. W. Clarke, *The Data of Geochemistry*, in *United States Geological Survey Bulletin*, 49 (1911).
A. Rittmann, *Vulcani: attività e genesi*, Napoli 1944; id., *Über die Herkunft der vulkanischen Energie und die Entstehung des Sials*, in *Geol. Rundschau*, 30 (1938), p. 52; W. Kuhn and A. Rittmann, *Über den Zustand des Erdinnern und seine Entstehung aus einem homogenen Urzustand*, ibid., 30 (1941), p. 215; W. Kuhn, *Stoffliche Homogenität des Erdinnern*, in *Naturwissenschaften*, 30 (1942), p. 689; H. Haberland, *Die Bedeutung der Spurenelemente in der geochemischen Forschung*, in *Monatshefte für Chemie*, 77 (1946), p. 293; G. Carobbi, *Proposte per un sistema cristallochimico degli ioni*, in *Atti e memorie dell'Accademia di Scienze, lettere ed arti di Modena*, 5th series, 7 (1947); H. Haberland, *Über die gesetzmässige Differentiation von Spurenelementen in Mineralien*, in *Tschermaks mineralogische und petrographische Mitteilungen*, 1 (1948), p. 134. Guido Carobbi.